Hydraulic tensioners have been widely used in the timing transmissions of automobile engines to maintain proper tension in the timing chain or belt, and to suppress vibration.
As shown in FIGS. 10 to 12, in a conventional hydraulic tensioner 500, a plunger 520 is slidable in a plunger-accommodating hole 511 formed in a housing 510. The plunger 500 is formed with a cylindrical hollow portion 521 with one end open. The hollow portion receives a plunger-biasing spring 530, which urges the plunger 520 in the protruding direction.
The housing and the plunger together form a high pressure chamber R. A check valve unit 540 extending from the housing into the high pressure oil chamber at one end thereof comprises a ball seat 541 having a tapered ball-engaging opening 541b, which serves as a valve seat, a check ball 542 facing the ball seat 541, a ball biasing spring 543, which biasses the check ball 542 toward the ball seat 541, and a retainer 544, which supports the ball biasing spring 543. A through hole 544a is formed at the peak end of the retainer 544, through which, oil in the high pressure chamber R is replenished from an oil passage 541a in the ball seat 541. The hydraulic tensioner of FIGS. 10-12 is described in United States Patent publication 2002/0142871, published Oct. 3, 2002.
FIGS. 13-15 show another conventional hydraulic tensioner 600, in which holes 644a are formed in a side surface of a retainer 644 in a check valve unit 640, for replenishing oil in a high pressure chamber R from an passage 641a in a ball seat 641 to R. The hydraulic tensioner of FIGS. 13-15 is described in U.S. Pat. No. 7,001,295, granted on Feb. 21, 2006.
In a hydraulic tensioner disclosed in United States Patent publication 2002/0142871, as shown in FIG. 11, the retainer 544 of the check valve unit 540 is attached to the ball seat 541 and is pressed against the bottom of the plunger-accommodating hole 511 by the plunger-biasing spring 530. When the plunger 520 slides rapidly with changes in tension in a timing chain, the retainer 544 can shift its position along the ball seat 541. As a result, the movement of the check ball 542 is not reliably restricted by the retainer 544, and the check valve ceases to function reliably.
Furthermore, as can be seen in FIG. 11, when the plunger 520 is pressed back into the plunger-accommodating hole 511, the plunger-biasing spring 530, located in the gap between the plunger 520 and the retainer 544, impedes the flow of oil from the high pressure chamber R to the outside of the tensioner through the leakage path between the plunger and the internal wall plunger-accommodating hole 511. As a result, rapid changes in tension in the timing chain cannot be absorbed instantaneously.
Furthermore, as shown in FIG. 12, oil in the high pressure chamber R, is replenished by oil from the oil passage 541a of the ball seat 541 through a circular through hole 544a in the peak of the retainer 544. In this check valve, if excessive oil is supplied from the external oil supply, the check ball 542 can close the through hole 544a, cutting off the oil supply to the high pressure chamber R.
The hydraulic tensioner disclosed in U.S. Pat. No. 7,001,295, has a problem similar to the problem exhibited by the tensioner in FIGS. 10-12. As shown in FIGS. 13-15, the retainer 644 of the check valve unit 640 (FIG. 13) is attached to the ball seat 641 and is pressed against the bottom of the plunger-accommodating hole 611 in tensioner housing 610 by the plunger-biasing spring 630. The check ball 642 is biased by a spring 643 into engagement with a tapered surface 641b formed at the end of the opening 641a of the ball seat 641. When the plunger 620 slides rapidly with changes in tension in a timing chain, the retainer 644 shifts its position along the ball seat 641. As a result, the movement of the check ball 642 is not reliably restricted by the retainer 644, and the check valve ceases to function reliably.
Furthermore, as shown in FIG. 14, when the plunger 620 is pressed back into the plunger-accommodating hole 611, the plunger-biasing spring 630, which extends into the hollow portion 621 of the plunger, and part of which is located in the gap between the plunger 620 and the retainer 644, impedes the flow of oil from the high pressure chamber R to the outside of the tensioner through the leakage path between the circumferential surface of the plunger and the internal wall of the plunger-accommodating hole 611. As in the case of the tensioner of FIGS. 10-12, rapid changes in tension in the timing chain cannot be absorbed instantaneously.
As shown in FIG. 15, since a plurality of holes 644a on the side of the retainer are provided to replenish oil to the high pressure chamber R from the oil passage 641a in the ball seat 641, when excessive oil is supplied from the external oil supply through the ball seat and holes 644a to the high pressure chamber R, changes in the tension of the timing chain are generated.
The plunger-biasing spring 630 in the gap between the plunger 620 and the retainer 644 impedes flow of oil from the high pressure chamber R through the gap between the plunger and the wall of the plunger-accommodating hole 611 to the outside of the housing. An increase in tension in the timing chain causes the plunger 620 to be pressed back into the plunger-accommodating hole 611, and, since leakage of oil to the outside the housing is impeded by the plunger-biasing spring 630, the pressure inside of the high pressure chamber R becomes abnormally high, and the plunger 620 does not move as it should.
Accordingly, an object of the invention is to solve the above-mentioned problems and to provide a hydraulic tensioner in which a bell-shaped retainer and a ball seat are reliably incorporated and fixed into check valve unit, the check valve unit functions reliably, and oil from the external oil supply oil leaks between the plunger and the wall of the plunger-accommodating hole to the outside of the housing without being influenced by the plunger-biasing spring.